“We used stellar evolution models to estimate the end of a planet’s habitable lifetime by determining when it will no longer be in the habitable zone. We estimate that Earth will cease to be habitable somewhere between 1.75 and 3.25 billion years from now,” Andrew Rushby, from UEA’s school of Environmental Sciences and the leader of the research said on the UEA website. “After this point, Earth will be in the ‘hot zone’ of the sun, with temperatures so high that the seas would evaporate. We would see a catastrophic and terminal extinction event for all life.

And life doesn’t necessarily mean humans — it can mean things as simple as micro-organisms.

“Of course conditions for humans and other complex life will become impossible much sooner — and this is being accelerated by anthropogenic climate change,” Rushby wrote. “Humans would be in trouble with even a small increase in temperature, and near the end only microbes in niche environments would be able to endure the heat.”

Rushby said that the most important part of figuring out the total habitable time for a planet is that it gives an idea of how long it takes for complex life to develop.

“Looking back a similar amount of time, we know that there was cellular life on earth. We had insects 400 million years ago, dinosaurs 300 million years ago and flowering plants 130 million years ago. Anatomically modern humans have only been around for the last 200,000 years — so you can see it takes a really long time for intelligent life to develop,” he wrote on his school site. “The amount of habitable time on a planet is very important because it tells us about the potential for the evolution of complex life — which is likely to require a longer period of habitable conditions.”

This, in turn, can help us search for how complex life would develop on other planets. Scientists are looking for an Earth-size planet that’s in the habitable zone or the so-called “Goldilocks” zone — that sweet spot that’s not too hot and not too cold where water, which is essential for life as know it, could exist on the surface.

“Looking at habitability metrics is useful because it allows us to investigate the potential for other planets to host life, and understand the stage that life may be at elsewhere in the galaxy.”

The Earth is actually near the outer edge of the habitable zone. Scientists say that it would be much more likely for complex life to exist on planets that are closer to the sun than us than further from it, though the strip of “Goldilocks” space, in intrastellar terms, is quite small.

“Interestingly, not many other predictions based on the habitable zone alone were available, which is why we decided to work on a method for this. Other scientists have used complex models to make estimates for the Earth alone, but these are not suitable for applying to other planets,” Rushby wrote.

In April, NASA unveiled new planetary results from its Kepler mission, showing two very Earth-like planets.

“Two of the newly discovered planets orbit a star smaller and cooler than the sun. Kepler-62f is only 40% larger than Earth, making it the exoplanet closest to the size of our planet known in the habitable zone of another star,” NASA explains in a release. “Kepler-62f is likely to have a rocky composition. Kepler-62e orbits on the inner edge of the habitable zone and is roughly 60% larger than Earth.”

The distant duo are the best candidates for habitable planets that astronomers have found so far, said William Borucki, the chief scientist for NASA’s Kepler telescope. Both are Earth-sized and in the habitable zone.

Another key planet, Kepler 22b, was unveiled on December 5, 2011. It’s 2.4 times the size of the Earth, orbiting a Sun-like star every 290 days. Another, Gliese 581d, was discovered around the same time.

“One of the planets that we applied our model to is Kepler 22b, which has a habitable lifetime of 4.3 to 6.1 billion years. Even more surprising is Gliese 581d which has a massive habitable lifetime of between 42.4 to 54.7 billion years. This planet may be warm and pleasant for 10 times the entire time that our solar system has existed!” Rushby wrote.

The planets were discovered by NASA’s Kepler space telescope, which measures fluctuations in the brightness of more than 150,000 stars in order to detect planets. Scientists then used ground-based telescopes to peer at the information the spacecraft has gathered in order to analyze and verify its discoveries.

However, none of the discovered planets are perfect Earth analogues, Rusby wrote.

“To date, no true Earth analogue planet has been detected. But it is possible that there will be a habitable, Earth-like planet within 10 light-years, which is very close in astronomical terms. However reaching it would take hundreds of thousands of years with our current technology.”

He says that the best bet to transplant the human race remains right next door. On Mars.

“If we ever needed to move to another planet, Mars is probably our best bet. It’s very close and will remain in the habitable zone until the end of the Sun’s lifetime — six billion years from now.”